How did john dalton discover the atomic theory?

He discovered it by Experiments and that led John Dalton to finding his theory !

Explanation:

Experiments with gases that first became possible at the turn of the nineteenth century led John Dalton in 1803 to propose a modern theory of the atom based on the following assumptions. Atoms of different elements combine in simple whole numbers to form compounds, 5. Atoms cannot be created or destroyed.

ℍ : Dalton discovered that oxygen combined with either one or two volumes of nitric oxide in closed vessels over water and this pioneering observation of integral multiple proportions provided important experimental evidence for his incipient atomic ideas. Ps:  I usally dont answer but I tried :D !